The present invention relates to a surface acoustic wave (SAW) device incorporating diamond, and particularly to a SAW device that has excellent operational performance even at frequency ranges such as gigahertz and higher-frequency bands.
As stated in the published Japanese patent application Tokukaihei 10-276061, a typical SAW device incorporating diamond is known to be produced by forming a ZnO layer on a diamond layer, forming on the ZnO layer interdigital electrodes (IDTs), which excite and receive SAWs, and finally forming an SiO2 layer on the ZnO layer such that the SiO2 layer covers the IDTs.
The SAW device is intended to accomplish not only excellent propagation, electromechanical coupling, and frequency-temperature properties but also low propagation loss by obtaining an optimum combination of the thicknesses of the IDTs, ZnO layer, and SiO2 layer. The SAW device achieves a frequency temperature property of xe2x88x9215 to +15 ppm/xc2x0 C. and an electromechanical coupling coefficient of 0.1 to 1.3% at a propagation velocity of 8,000 to 12,000 m/s.
However, when the implementation of the conventional SAW device is planned for use at a frequency band as high as 10 GHz or so, even if the propagation velocity is increased to 10,000 m/s, it is necessary to reduce the sum of the width of the digit electrode and the distance between the neighboring digit electrodes of the IDTs to 0.5 xcexcm or so and the width of the digit electrode to 0.25 xcexcm or so. This requirement is disadvantageous for mass production of the SAW device.
Moreover, a conventional material such as quartz has a limited propagation velocity of 3,150 m/s, and therefore, cannot be used for a SAW device for the superhigh-frequency band.
The conventional SAW device has another drawback in that the electromechanical coupling coefficient decreases at the superhigh-frequency band. In particular, the electromechanical coupling coefficient at the third harmonic varies with the metal ratio of the digit electrode portion of the IDTs. When the metal ratio is 0.5, the electromechanical coupling coefficient becomes zero. Here, the metal ratio represents the ratio dm/(dm+df), where dm is the width of the digit electrode and df is the distance between the neighboring digit electrodes. For example, a SAW device made with quartz, which has an electromechanical coupling coefficient of 0.1% at the fundamental wave, reduces the coefficient to 0.03% at the third harmonic when the metal ratio is 0.3. To achieve a center frequency of 10 GHz requires the value of xe2x80x9cdm+dfxe2x80x9d to be 0.47 mm and the value of dm to be 0.14 mm even at the third harmonic. This requirement makes it difficult to mass-produce SAW devices. An extension of the band width of the operating frequency requires an increase in the electromechanical coupling coefficient. This increase requires a further reduction in the metal ratio, making mass production of the SAW device increasingly difficult.
The present invention aims to solve the foregoing problems, and its object is to offer a SAW device that is suitable for mass production and that has excellent operational performance at the superhigh-frequency range.
A SAW device of the present invention comprises:
(a) a diamond layer;
(b) a ZnO layer, with a thickness of tz, formed on the diamond layer;
(c) IDTs, which excite and receive a SAW, formed on the ZnO layer; and
(d) an SiO2 layer, with a thickness of ts, formed on the ZnO layer so that the SiO2 layer can cover the IDTs.
In order to determine the structure of the SAW device, a two-dimensional orthogonal-coordinate system is provided, in which the axis of abscissa represents kh1 and the axis of ordinate represents kh2. In the above description, kh1 and kh2 are given in the following equations:
kh1=3xc2x72xcfx80xc2x7(tz/xcex); and 
kh2=3xc2x72xcfx80xc2x7(ts/xcex), 
where xcex signifies the wavelength of the fundamental wave of the second Sezawa mode of the SAW.
In the orthogonal-coordinate system, the range ABCDEFGHIJKLA is provided by connecting sequentially the following 12 points with 12 lengths of lines:
point A given by the coordinates xe2x80x9ckh1=0.30 and kh2=0.87xe2x80x9d;
point B given by the coordinates xe2x80x9ckh1=0.54 and kh2=0.87xe2x80x9d;
point C given by the coordinates xe2x80x9ckh1=0.60 and kh2=0.87xe2x80x9d;
point D given by the coordinates xe2x80x9ckh1=0.81 and kh2=0.97xe2x80x9d;
point E given by the coordinates xe2x80x9ckh1=1.16 and kh2=1.20xe2x80x9d;
point F given by the coordinates xe2x80x9ckh1=1.52 and kh2=0.93xe2x80x9d;
point G given by the coordinates xe2x80x9ckh1=1.69 and kh2=0.77xe2x80x9d;
point H given by the coordinates xe2x80x9ckh1=1.31 and kh2=0.59xe2x80x9d;
point I given by the coordinates xe2x80x9ckh1=1.04 and kh2=0.50xe2x80x9d;
point J given by the coordinates xe2x80x9ckh1=0.68 and kh2=0.40xe2x80x9d;
point K given by the coordinates xe2x80x9ckh1=0.63 and kh2=0.33xe2x80x9d;
point L given by the coordinates xe2x80x9ckh1=0.30 and kh2=0.63xe2x80x9d; and
point A
The combination of kh1 and kh2 is determined so that it can fall in the range ABCDEFGHIJKLA including the surrounding 12 lengths of lines. The SAW device uses the third harmonic of the second Sezawa mode of the SAW. The third harmonic of the second Sezawa mode of the SAW has a propagation velocity, denoted by v, of 8,000 to 12,000 m/s. The IDTs have a plurality of comb-tooth-shaped digit electrodes of which:
(a) the sum of the width of a digit electrode and the distance between the neighboring digit electrodes is expressed as
dm+df=(3xc2x7v)/(2xc2x7f0), 
where dm: the width of the digit electrode,
df: the distance between the neighboring digit electrodes, and
f0: the center frequency of the third harmonic of the second Sezawa mode of the SAW;
(b) the sum of xe2x80x9cdm+dfxe2x80x9d is 1.0 xcexcm or more; and
(c) the metal ratio xe2x80x9cdm/(dm+df)xe2x80x9d satisfies the formula 0.15 less than dm/(dm+df) less than 0.3 or the formula 0.7 less than dm/(dm+df) less than 0.85. The center frequency of the third harmonic of the second Sezawa mode of the SAW is 5 to 12 GHz.
A saw device of the present invention may have the IDTs having the digit electrodes of which the sum xe2x80x9cdm+dfxe2x80x9d is 1.2 to 1.8 xcexcm. In this case, the center frequency of the third harmonic of the second Sezawa mode of the SAW is 9.5 to 10.5 GHz.
As mentioned above, the present invention uses the third harmonic of the second Sezawa mode of the SAW excited by the IDTs. This enables the SAW device to obtain an excellent propagation property, electromechanical coupling coefficient, and frequency-temperature property at the superhigh-frequency range. Moreover, the present invention allows the use of wider digit electrodes in the IDTs, so that mass production of the SAW device can be easily achieved.